Conjugate addition of potassium trifluoro(organo)borates 2 to dehydroalanine derivatives 1, mediated by a chiral rhodium catalyst and in situ enantioselective protonation, afforded straightforward access to a variety of protected alpha-amino esters 3 with high yields and enantiomeric excesses up to 95%. Among the tested chiral ligands and proton sources, Binap, in combination with guaiacol (2-methoxyphenol), an inexpensive and nontoxic phenol, afforded the highest asymmetric inductions. Organostannanes have also shown to participate in this reaction. By a fine-tuning of the ester moiety, and using Difluorophos as chiral ligand, increased levels of enantioselectivity, generally close to 95%, were achieved. Deuterium labeling experiments revealed, and DFT calculation supported, an unusual mechanism involving a hydride transfer from the amido substituent to the alpha carbon explaining the high levels of enantioselectivity attained in controlling this alpha chiral center.

We report here our full results concerning the possibility of generating in situ from a stable and readily available ruthenium(II) source a highly active ruthenium catalyst for C-H bond activation. The versatility of this catalytic system has been demonstrated, as it offers the possibility of modifying the electronic and steric properties of the catalyst by fine-tuning of the ligands, allowing functionalization of various substrates. Aromatic ketones and imines could be easily functionalized by the reaction with either vinylsilanes or styrenes, depending on the electronic and steric nature of the ligand. Moreover, variable-temperature NMR experiments and the isolation of a ruthenium intermediate complex provided some insights into the generation of the active catalytic ruthenium species in this reaction.

Easily modified: the electronic and steric properties of a ruthenium catalyst highly active for CH bond activation (see scheme) can be modified by fine‐tuning the ligand. This makes this catalytic system very versatile as it allows functionalization of a variety of substrates. The catalyst is generated in situ from a stable and easily available ruthenium(II) source.